1. Field of the Invention
The present invention relates to an optical module applicable to an optical data link used for optical communications; and, in particular, to a structure for attaching a sleeve to a head portion in which an optical device such as light-receiving device or light-emitting device is molded with a resin.
2. Related Background Art
A conventional optical module comprises, at least a can-shaped TO (Transistor Outline) type standard package which incorporates an optical device such as light-emitting device or light-receiving device therein and is mounted with a condenser lens. To the TO type standard package, an alignment sleeve for receiving an optical fiber ferrule attached to a tip of an optical fiber acting as an optical transmission line has been secured with the aid of an adhesive (see U.S. Pat. No. 5,596,665).
Here, since the housing of the TO type standard package accommodating the optical device therein is made of a metal, the optical module tends to become expensive and larger in size, while being hard to be processed into a desirable form and thus being low in its freedom of design. Therefore, in place of such a metal package, optical modules of an integrated structure in which an optical device such as light-emitting device or light-receiving device directly mounted on a lead frame is molded with a plastic resin have been proposed (see U.S. Pat. Nos. 4,410,469 and 4,539,476).
Namely, as shown in FIG. 1, a conventional optical module comprises a head portion 2 (resin mold portion) molding an optical device mounted on a lead frame with a transparent resin, a body portion 4 (resin mold portion) molding an electronic device mounted on the lead frame with an opaque resin, and a plurality of lead pins 6 (connecting portion) for electrically and mechanically connecting these resin mold portions 2 and 4 to each other.
Further secured to a front end part 2a of the head portion 2 is an opaque cylindrical sleeve 8 for receiving an optical fiber ferrule. As the optical fiber ferrule is inserted into the cylindrical sleeve 8, the conventional optical module can automatically align the optical fiber and the optical device in the head portion 2 with each other in terms of optical axis.
The operation of securing the cylindrical sleeve 8 to the head portion 2 has been carried out such that, as shown in FIG. 2, the front end part 2a of the head portion 2 is inserted into an insertion hole 8a formed at the rear end part of the cylindrical sleeve 8, the gap between the inner wall face of the insertion hole 8a and the surface of the front end part 2a is filled with a UV-curable resin 100, and then the UV-curable resin 100 is cured upon irradiation with ultraviolet rays UV. Alternatively, the securing operation may be carried out such that a thermosetting resin is injected into the above-mentioned gap in place of the UV-curable resin, and then thus injected resin is cured upon heating.
Having studied the above-mentioned conventional art from various viewpoints, the inventors have found the following problems.
1) First, when using a UV-curable resin for bonding the sleeve, the irradiation efficiency of ultraviolet rays must be taken into consideration.
As shown in FIGS. 1 and 2, the head portion 2 has a structure in which the front end part 2a shaped like a circular truncated cone and a rectangular base 2b larger than the front end part 2a are integrally molded by transfer molding. Consequently, when securing the cylindrical sleeve 8 to the head portion 2 with the aid of the UV-curable resin 100, these members are irradiated with ultraviolet rays UV in the state where the base 2b abuts to the rear end part of the cylindrical sleeve 8. Here, since the cylindrical sleeve 8 is opaque to the ultraviolet rays UV, it is necessary for the ultraviolet rays UV to be emitted toward the gap between the insertion hole 8a of the cylindrical sleeve 8 and the front end part 2a of the head portion 2 from the rear side of the base 2b. 
In practice, however, the gap filled with the UV-curable resin 100 is blocked with the base 2b, thereby the UV illuminance would inevitably decrease even when the head portion 2 is a resin mold body transparent to the ultraviolet rays. As a consequence, it has taken a long time to fully solidify the UV-curable resin 100.
2) On the other hand, when using a thermosetting resin for bonding the sleeve, attention has to be paid to the handling of the excess part of the filling resin in particular.
Though the thermosetting resin introduced as an adhesive cures upon the heating processing after the injection, the thermosetting resin has such a characteristic that, with its viscosity once decreasing upon heating, it spreads out into the gap between the inner wall face of the sleeve 8 and the front end part 2a before curing. As a consequence, when the viscosity of the thermosetting resin once decreases, there is a possibility of the excess thermosetting resin flowing into a ferrule insertion hole 8b positioned on the opposite side of the insertion hole 8a or adhering to the condenser lens 2c mounted to the front end part 2a. This phenomenon is not negligible since the amount of injection of resin cannot be reduced in order to attain a sufficient bonding strength.
For example, if the excess thermosetting resin that has flowed into the insertion hole 8b cures, then, upon inserting the ferrule therein, the optical fiber received therein and the condenser lens will shift from each other in terms of optical axis. On the other hand, the excess thermosetting resin attached to the surface of the condenser lens 2c causes the optical function of the condenser lens 2c to deteriorate, thereby the optical module would lower its optical characteristics. In particular, when the front end part 2a and the insertion hole 8a of the sleeve 8 are made smaller in response to the demand for reducing the size of the optical module, then the gap between the side wall of the front end part 2a and the inner wall face of the sleeve 8 becomes narrower. As a consequence, the thermosetting resin with its viscosity lowered upon heating is more likely to spread out due to the capillary action, thereby flowing into the deep part of the insertion hole 8b or adhering to the surface of the condenser lens 2c by a large amount.
3) In addition, when taking account of the environment where the optical module is actually used, it is preferred that the adhesion durability of the sleeve be improved.
As a method of molding a resin mold portion (head portion 2 or body portion 4) or sleeve in conventional modules, it is common to use transfer molding in which a plastic resin is injected into a die having a cavity with a predetermined form and is molded therein. The surface of thus obtained resin mold portion or sleeve is processed as smooth as possible in view of die-cutting or the like.
In the case where a sleeve is secured to such a resin mold portion with the aid of an adhesive such as UV-curable resin, smooth surfaces inevitably face each other with the adhesive interposed therebetween. As a consequence, it has been difficult to improve durability in the conventional optical modules against expected changes in their use environment (e.g., temperature changes within the range from xe2x88x9240xc2x0 C. to +85xc2x0 C., moisture changes, and the like).
It is an object of the present invention to provide an optical module comprising a variety of structures for overcoming the problems mentioned above.
The optical module according to the present invention is applicable to an optical data link for connecting an optical fiber transmission line and an electric signal transmission line to each other and comprises an integrated structure constituted by a head portion molding an optical device such as light-emitting device or light-receiving device as a whole with a resin and an alignment sleeve attached thereto. This sleeve is secured to the head portion, in a state accommodating at least a part of the head portion therein, with an adhesive interposed therebetween.
A first embodiment of the optical module according to the present invention comprises a structure for improving the working efficiency when a UV-curable resin such as that mentioned above is used. Namely, the optical module according to the first embodiment comprises a head portion molding an optical device as a whole with a resin; and, a hollow sleeve, extending along a predetermined reference axis, provided with an opening for accommodating at least a part of the head portion therein. The sleeve, in a state accommodating a part of the head portion therein via the opening thereof, is secured to the head portion with an adhesive interposed therebetween.
In particular, in the optical module according to the first embodiment, the part of the head portion accommodated in the sleeve has a side face tilted with respect to the reference axis by a first angle, whereas the remaining part of the head portion exposed from the sleeve has a side face tilted with respect to the reference axis by a second angle which is equal to or smaller than the first angle. Preferably, of the head portion, the remaining part exposed from the sleeve has a cross-sectional area orthogonal to the reference axis equal to or smaller than the cross-sectional area of the head portion matching the opening face of the sleeve. Here, the head portion is designed such that the maximum value of the cross-sectional area of the head portion orthogonal to the reference axis is smaller than the opening area of the sleeve.
When the part of the head portion (the rear end part of the head portion) exposed from the sleeve has a cross-sectional area equivalent to or smaller than the maximum cross-sectional area of the head portion, then the gap filled with the UV-curable resin can be exposed to a UV source. Consequently, the ultraviolet rays from the UV source directly irradiate the filling UV-curable resin.
A second embodiment of the optical module according to the present invention comprises a structure for effectively preventing optical characteristics from deteriorating when a thermosetting resin such as that mentioned above is used. As with the optical module according to the first embodiment, the optical module according to the second embodiment comprises a head portion molding an optical device as a whole with a resin; and, a sleeve, extending along a predetermined reference axis, secured to the head portion, in a state accommodating at least a front end part of the head portion therein, with an adhesive interposed therebetween.
In particular, the optical module according to the second embodiment further comprises a restriction structure, provided in at least one of the sleeve and head portion, for restricting a flow of the adhesive injected between the sleeve and the front end part of the head portion accommodated in the sleeve. As the flow of excess adhesive is restricted, the adhesive is prevented from unnecessarily spreading out.
The restriction structure can be realized when the inner wall face of the sleeve mating with the front end part of the head portion is provided with a depression for reserving the excess part of adhesive flowing upon heating. Preferably, a surface of the head portion is provided with a lens body positioned on the reference axis, and the depression is arranged such as to surround the lens body continuously or intermittently.
Alternatively, the restriction structure may be provided on the head portion side. Namely, the restriction structure can also be realized when the front end part of the head portion accommodated in the sleeve is provided with a restriction protrusion continuously or intermittently surrounding the lens body attached to the surface of the front end part of the head portion. Here, the method of introducing the adhesive for filling the gap between the sleeve and the head portion is not limited to the introduction via the opening of the sleeve. Namely, when the sleeve is provided with a plurality of through holes communicating the outer peripheral surface of the sleeve and the accommodation space for the front end part to each other, then the adhesive can be introduced via these plurality of through holes as well.
A third embodiment of the optical module according to the present invention comprises a structure for improving the adhesion durability of the sleeve in a bonding operation such as that mentioned above. As with the optical modules according to the first and second embodiments, the optical module according to the third embodiment comprises a head portion molding an optical device as a whole with a resin; and, a sleeve, extending along a predetermined reference axis, secured to the head portion, in a state accommodating at least a front end part (first protrusion) of the head portion therein, with an adhesive interposed therebetween.
In particular, for improving the adhesion durability between the front end part and the sleeve in the head portion in the optical module according to the third embodiment, at least one of the sleeve and head portion is provided with a bonding strength enhancing structure for enhancing the bonding strength between the sleeve and the head portion.
Specifically, the bonding strength enhancing structure can be realized when at least the sleeve, mainly at the inner wall face thereof mating with the front end part of the head portion, is provided with a thread groove. This thread groove does not only yield an anchor effect as the adhesive pervades there. When a thread groove is also formed in the front end part surface of its opposing head portion, the position of the ferrule facing the head portion with the aid of the sleeve can be adjusted. Namely, engaging the respective thread grooves formed in the sleeve and front end part surface of the head portion with each other enables the positional adjustment of the sleeve along its center axis, thereby the distance between the end face of the optical fiber supported by the ferrule and the optical device within the head portion can be controlled accurately.
The bonding strength enhancing structure can also be realized when at least one of the inner wall face of the sleeve and the front end part surface of the head portion accommodated in the sleeve is provided with a protrusion pattern having a predetermined height formed by surface processing such as etching.
Though the surface processing may be carried out after the resin molding of the sleeve or head portion; since a superfluous manufacturing step is added thereby, it is more preferred that the inner wall face or front end part surface be embossed. As the adhesive pervades the embossed surface, the bonding strength between the sleeve and the front end part increases. The embossment is constituted by a plurality of depressions having a maximum depth of about 10 xcexcm, preferably within the range of 10 xcexcm to 20 xcexcm. It is due to the fact that a sufficient bonding strength may not be obtained when the depressions are too shallow, whereas the sleeve and the like may not be pulled out of the die after the resin molding when the depressions are too deep. The bonding strength between the sleeve and the head portion depends not only on the depth of the depressions constituting the embossment but also on the form of the embossed surface. As a consequence, the embossment formed in the optical module according to the present invention in order to attain a desirable bonding strength is constituted by a plurality of depressions having a maximum diameter of 20 xcexcm to 50 xcexcm, while the depressions constituting the embossment occupy about 80%, preferably 60% to 90%, of the embossed surface.
Further, the bonding strength enhancing structure can also be realized by providing an auxiliary wall (second protrusion) surrounding the front end part of the head portion, and bonding and securing the sleeve to the head portion in a state where the opening part of the sleeve is held between the front end part and the auxiliary wall. In this structure, the bonding areas in the head portion and in the sleeve would be nearly twice as much as those in the conventional optical module, thereby the bonding strength would also improve remarkably as compared with that conventionally attained. Also, in the optical module according to the third embodiment, a structure in which a protrusion such as an auxiliary wall is formed near the front end part of the head portion and a structure in which the sleeve side or head portion side is subjected to surface processing such as embossing can be combined together.